The present invention relates to a method and an apparatus for dividing the flow of a fiber suspension in the pulp and paper industry into partial streams.
The pulp and paper industry often requires that a fiber suspension is conveyed from a vessel or a pipe continuously or intermittently, divided evenly or unevenly, to several places, e.g. from a storage tank simultaneously to two or more processing devices. When the consistency of the suspension is low, i.e. up to about 5 percent, dividing of the flow itself does not cause any problems, but when the consistency is higher than about 5% there is only a small amount of free liquid between the fibers and the fibers form a strong fiber network. The strength of the network depends on the consistency. The characteristics of medium or high consistency fiber suspensions are quite different from that of a true liquid and the handling thereof becomes more difficult the higher the consistency is.
As stated, when the consistency is high, e.g. from about 8 to about 25 percent, the fiber suspension forms a strong fiber network and dividing the fiber suspension in a pipeline or other conduit is mostly impossible without special measures. When a high-consistency fiber suspension arrives, usually in form of a plug flow, at a junction point in the pipeline, the fiber network is too strong to be dispersed. The rigid fiber network will also adhere to a part of the pipe resulting in clogging of the pipeline. When one branch of the pipeline is not in use i.e. it is closed by a valve, the portion of the pipe preceding the valve is readily clogged and will not open up when the valve is opened as the friction is the highest while the suspension is at rest and the line pressure is insufficient to push the fiber plug forward.
The above-described problem in dividing the pulp flow is avoided by subjecting the flow to a field of shear forces of sufficient strength so that the bonds between the fibers are loosened or broken and instead an at least partially turbulent flow is created in which there exist no fiber networks that prevent the division of the suspension. In low consistency fiber suspension (below about 5%) the shear forces may be created by the geometry of the junction point, while in high consistency fiber suspension, (above about 5%) sufficient shear forces are generated by a rotor arranged at or in the vicinity of to the junction point.
In many practical applications the above described dividing apparatus, with or without a rotor, has been placed in a conduit closely after or downstream of the pump. The pressure drop in the dividing apparatus, especially in the one having no rotor, is so high, particularly when the consistency of the pulp suspension is high, that the pressure created by the pump is almost totally lost in the dividing apparatus.
Up to now, there have been known devices designed solely for dividing the flow of fiber suspension. In other words the known devices must be located in a pipeline downstream of a pump that is creating the pressure difference required for causing the pulp to flow through the dividing apparatus. However, in this known apparatus pressure loss occurs in the pipeline. The known apparatus is complex and wastes the more energy the higher the consistency of the pulp is. At this stage it is worth noticing that in many cases the pulp transfer distances are quite short in a pulp mill so that only a slight increase in pulp pressure is needed to ensure a continuous pulp flow, pipe junctions or dividing apparatus excluded. It thus appeared advantageous to create said pressure difference in the dividing apparatus between the inlet and outlet connections of the junction point for facilitating the dividing of the flow towards several outlets and for ensuring the flow of the pulp to the next treatment apparatus.